Necked nonwoven webs, including necked spunbond webs, meltblown webs, combinations, and the like, are often made using a process which is schematically illustrated in FIG. 1. A nonwoven web 12 having a starting width A is passed in its machine direction between a first nip 16, which can be a first pair of nip rollers traveling at a first surface velocity, and a second nip 26, which can be a second pair of nip rollers traveling at a second surface velocity which is faster Man the first surface velocity. The surface velocity difference between the first and second nips results in formation of a narrower (“necked”) nonwoven web 22 having a necked width A′ which is less than the starting width A. The second average surface velocity is about 1.05-1.7 times the first average surface velocity, suitably about 1.1-1.5 times the first average surface velocity, desirably about 1.2-1.4 times the first average surface velocity.
The necked nonwoven web 22 generally includes fibers which are closer together and more aligned in the machine direction than the fibers of the starting nonwoven web 12, which can be more randomly aligned. While compacting and aligning the fibers, the necking process generally does not stretch individual fibers. The necking may be performed with the aid of heat applied below the melting temperature of the fibers, for instance, by placing an oven or other heat source between the first and second nips. The necked nonwoven web 22 may also be heat set, either during or after the necking process, so that the necked web becomes somewhat stable. A nonwoven web which is stable in the necked condition is said to be “reversibly necked.” A reversibly necked nonwoven web can be easily extended in the cross direction by applying a small extension force, and tends to return to its narrower, necked configuration when the extension force is released.
The starting nonwoven web 12 includes edge regions 13 and 15, and a central region 11. The necked nonwoven web 22 includes edge regions 23 and 25, and a central region 21. Because the necking causes the nonwoven fibers to become closer together and more aligned, without noticeably stretching or narrowing the individual fibers, the necked nonwoven web 22 generally has a higher basis weight than the starting nonwoven web 12.
As can be easily seen from FIG. 1, the nonwoven fibers in the edge regions 13 and 15 of the starting nonwoven web are subject to different strain, and travel a greater distance between the first nip 16 and the second nip 26 of the necking process, than the fibers in the central region 11. Furthermore, the cross-directional stresses in the central region 11 are at least partially counteracted, because these stresses are applied in both cross directions. The cross-directional stresses in each of the edge regions 13 and 15 are primarily in one direction, inward toward the center of the web. This results in increased fiber gathering and necking in the edge regions. Consequently, the fibers in the edge regions 23 and 25 of the necked nonwoven web are generally more aligned and closer together than the fibers in the central region 21. As a result, the necked nonwoven web may be nonuniform in the cross direction, having a higher basis weight in both edge regions than in the central region, and having greater cross-directional extendibility in both edge regions than in the central region.
There is a need or desire for neckable nonwoven materials which produce necked nonwoven webs having better cross-directional uniformity. There is also a need or desire for necked nonwoven webs, and laminates containing necked nonwoven webs, which have better cross-directional uniformity.